1. Field of the Invention
This invention relates to an edge enhancement apparatus for a video signal.
2. Description of the Prior Art
An edge enhancement apparatus for a liquid crystal display (LCD) has been known, which enhances an input video signal in level at edge portions to improve response or increase sharpness.
FIG. 7 is a block diagram of a prior art edge enhancement apparatus for a video signal for a liquid crystal display. FIG. 8 shows waveforms at each output point of the prior art edge enhancement apparatus. In FIG. 7, an input video signal is sifted in phase by two delay circuits 3 and 4 and then an edge enhancement signal Sd" is obtained at a subtractor 25. Such circuit is also known as a cosine filter. The edge enhancement signal Sd" is added to the input video signal to obtain edge enhanced video signal which is supplied to a liquid crystal display. Such edge enhancement apparatus of a prior art enhances the video signal at edge portions. However, there is a problem that picture quality is deteriorated at edge portions because a unnatural highlight and dark lines are developed near a outline of a picture image. This is because this prior art edge enhancement apparatus develops undershoot and overshoot at an edge portion, as shown by an waveform of Se". Further, in the liquid crystal display, the prior art edge enhancement apparatus does not provide the practical effect sufficiently in edge enhancement because generally, a dynamic rage of the LCD cannot be expected to be large and thus, undershoot and overshoot does not contribute edge enhancement sufficiently.